1. Technical Field of the Invention
The present invention relates generally to an improved structure of a device for measuring an angular position of an object using a magnetic detector and magnets, and more particularly to a compact and high-accuracy angular position measuring device.
2. Background Art
U.S. Pat. No. 5,861,745 (Japanese Patent First Publication No. 2920179, PCT:WO98/080601) discloses an angular position measuring device using a magnetic detector such as a Hall element and magnets.
FIG. 19 shows one example of conventional angular position measuring devices designed to measure an opened position of a throttle valve of an internal combustion engine. The device includes a cylindrical rotor core (i.e., a yoke) 11 rotating along with the throttle valve (not shown) and a stator core 12 disposed coaxially within the rotor core 11. Disposed between the rotor core 11 and the stator core 12 are two arc-shaped magnets 13 which are diametrically opposed to each other. Each of the magnets 13 is so magnetized that all lines of magnetic force are oriented in a radius direction of the magnet 13. A magnetic gap 14 is formed in the stator core 12 which has a constant width and extends through the center of the stator core 12 in a diameter direction. A magnetic detector such as a Hall IC is disposed at the center of the magnetic gap 14.
The density of a magnetic flux through the magnetic gap 14 in the stator core 12 changes as a function of an angular position of the rotor core 11. The magnetic detector 15 produces an output as a function of the magnetic flux density. Specifically, the angular position of the rotor core 11, or the opened position of the throttle valve is determined using the output of the magnetic detector 15.
The arc-shaped magnets 13 are, as described above, magnetized radially. Uniformly magnetizing the magnets 13 requires decreasing the magnetic flux density in an outer peripheral portion of the magnets 13, while increasing the magnetic flux density in an inner peripheral portion of the magnets 13. Such magnets are, however, difficult to produce and may have a great variation in quality. The variation in quality will lead to an error in an output of the magnetic detector 15.
Additionally, the installation of the magnets 13 between the rotor core 11 and the stator core 12 will result in an increase in diameter of the rotor core 11, thereby leading to an increase in overall size of the angular position measuring device.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a compact angular position measuring device capable of determining an angular position of a rotary object with high accuracy.
According to one aspect of the invention, there is provided an angular position measuring device which comprises: (a) a stationary member; (b) a rotatable member rotatable following rotation of an object to be measured in angular position; (c) a plurality of magnets installed in one of the stationary member and the rotatable member so that the same poles are opposed magnetically to produce a repellent force in magnetic fields of the magnets; and (d) a sensor element installed in the other of the stationary member and the rotatable member, the sensor element being responsive to a change in density of magnetic flux produced by the magnets to provide an output as a function of an angular position of the object.
In the preferred mode of the invention, each of the magnets is made of one of a plate and a cylindrical member whose ends are magnetized.
The rotatable member is made of a hollow cylindrical yoke having installed therein the magnets. The stationary member is made of a stator core having installed therein the sensor element and disposed inside the cylindrical yoke. A sensor gap is formed in the stator core within which the sensor element is disposed.
The cylindrical yoke may have air cavities formed in an inner surface thereof which open to the magnets for avoiding a short of the magnetic flux between each pole of the magnets and the stator core.
The magnets may be so arranged as to define two magnetic paths along which the magnetic fluxes produced by the magnets pass. The magnetic paths extend symmetrically through the stationary member and the rotatable member.
The stator core is made of three or more parts which are so fabricated as to define air gaps one of which is the sensor gap within which the sensor element is disposed.
The air gaps extend radially in the stator core at a regular angular interval. The magnets are arranged at an angular interval identical with the angular interval of the air gaps.
The stator core may be circular in cross section. The one of the air gaps employed as the sensor gap is longer than a radius of the stator core.
The one of the air gaps as employed as the sensor gap is longer than the other air gaps.
The stator core may have formed in at least one of ends of the sensor gap a greater air cavity which serves to concentrate the magnetic flux at the sensor gap.
An air gap may be defined between the inside of the cylindrical yoke and the outside of the stator core. The air gap is defined by an interval between the inside of the cylindrical yoke and the outside of the stator core which varies in a circumferential direction of the inside of the cylindrical yoke.
The interval may be maximized at each of the poles of said magnets.
The cylindrical yoke defines therein an oval chamber in which the stator core is disposed.